1. Field of the Invention
This invention relates to tunable filters generally and, more particularly, to an apparatus for tuning a filter disposed on an integrated circuit.
2. Description of the Prior Art
Servo filters arc used in the read channel of mass storage systems, such as magnetic disk media systems, to filter servo data patterns recorded on media to facilitate read/write head tracking. The servo data patterns are typically embedded in the data patterns recorded on the media. The filter, typically a low pass filter, has a cutoff frequency that is set for a given system.
It is desirable to implement the filter and its associated tuning circuitry in a common integrated circuit. Prior art tuning techniques typically rely on a phase-locked-loop (PLL) arrangement, such as that discussed in U.S. Pat. No. 4,509,019, assigned to the same assignee as this invention, and incorporated herein by reference. However, PLLs continuously dissipate power to maintain the filter's cutoff frequency even when the filter is not being used. If the reference signal (to which the PLL is locked) is removed, then the PLL drifts to a nominal frequency, changing the characteristic frequency of the filter along with it. This may be undesirable in many instances.
Thus, it may be desirable to maintain a desired filter characteristic without dissipating significant power when the filter is not being adjusted.
It is also desirable to have the filter characteristics remain substantially constant for relatively long periods without having to continually track the filter to a reference signal.